1. Field of the Invention
The present invention relates to an electric storage device, which includes an electric storage element having an external terminal and a bus bar connected to the external terminal, and a manufacturing method of the electric storage device. The present invention also relates to a bus bar to be connected to the external terminal of the electric storage element having the external terminal.
2. Description of the Related Art
In recent years, an electric storage element such as a battery (a lithium ion battery, a nickel-metal hydride battery, and the like) and a capacitor (an electric double layer capacitor and the like) which can be charged and discharged is employed as a power source of a vehicle (an automobile, a motorcycle, and the like) or various devices (a personal digital assistance, a notebook computer, and the like).
When high-volume electric power is required, an electric storage device including a plurality of electric storage elements and bus bars for electrically connecting the plurality of electric storage elements (see paragraph 0025 in JP-A-2010-160931) is employed as a power source.
Each of the electric storage elements includes a power generating element, a case for housing the power generating element, and external terminals disposed outside the case. The external terminals are electrically connected to the power generating element by shaft-shaped connecting bodies formed integrally with or separately from the external terminals. The electric storage element has gaskets disposed along an inner face and an outer face of the case. In this type of electric storage element, the connecting bodies pass through the case and the gaskets to electrically connect the power generating element in the case and the external terminals outside the case. The gaskets seal the holes which are formed in the case and through which the connecting bodies pass. Thus, an inside of the case is maintained airtight and liquid-tight.
There are various types of bus bars. An example of bus bars is welded to the external terminal(s) of the electric storage element(s). The bus bar has a main body portion and (a) connection portion(s) and is connected to the external terminal(s) through the connection portion(s).
More specifically, the bus bar is formed by a metal plate member having a substantially uniform thickness. The bus bar has the connection portion at least at one of its end portions. In other words, the bus bar having the main body portion and the connection portion(s) may have the connection portion connected to one end of the main body portion or may have the connection portions connected to both ends of the main body portion. The bus bar having the connection portion connected to the one end of the main body portion connects the external terminal of the electric storage element and an external electric load. The bus bar having the connection portions connected to the both ends of the main body portion connect external terminals of two electric storage elements.
In any case, the connection portion(s) is/are placed on (an) upper face(s) of the external terminal(s) of the electric storage element(s), and the connection portion(s) and the external terminal(s) (upper face(s)) are welded together. Thus, the bus bar is mechanically and electrically connected to the external terminal(s) of the electric storage element(s).
When the bus bar (connection portion(s)) and the external terminal(s) are welded together, a large amount of heat for melting the entire thickness of the bus bar is required. Therefore, in the conventional electric storage device, the gaskets may be softened or melted when the bus bar is welded. As a result, the gaskets cannot sufficiently seal the case, and the inside of the case cannot be maintained airtight and liquid-tight. Each of the external terminals is connected to the power generating element with a connecting conductor interposed therebetween. Therefore, the heat in welding the bus bar may be transferred to the inside of the case (e.g., to the power generating element or the like) through the external terminal. Therefore, the power generating element or the like in the case may be affected by the heat and its performance may deteriorate.